I find that integrated tests in software projects go a long way to reducing rot. When something eventually invariably breaks due to some external factor, the test suite greatly reduces the time to identify and fix the problem.
Without a good suite, the larger the system gets, the more people get scared of doing radical things to it because they are worried about breaking some critical functionality. As far as the not being able to build aspect goes, the main threat here is relying to much on closed source software and/or libraries that go out of support that no one can find the license, or critical documentation for, or worse manifests some critical unfixable bug.
But I suppose that's the problem; we don't have the time to fix bugs related to the environment that crop up when no code changes so we insulate our tests from the environment by design.